Porous particle aggregate and method therefor

ABSTRACT

A porous aggregate having a high intraaggregate reticular volume for releasable containment of functional substances is described. The porous aggregates comprise discrete particles, preferably starch granules, bound together at least at their points of contact in the aggregates. The surfaces of the discrete particles cooperate to define an intraaggregate reticulate volume. The porous aggregate composition of this invention finds use as a high capacity carrier of functional substances for a wide variety of applications, in which the functional substance is released from the aggregate composition under the influence of mechanical compression/disintegration, by degradation or dissolution of the binder and/or particulate components, or by diffusion from the porous surface.

This application is a divisional of Ser. No. 08/182,442, filed on Jan.14, 1994, now U.S. Pat. No. 5,486,507.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to particulate carriers for functionalsubstances. More particularly, this invention is directed to particleaggregates having high porosity and a large intraaggregate reticulatevolume for containment of functional substances. The present aggregatecompositions are economically manufactured to have predetermined releasecharacteristics and other desired physical properties.

There has been a significant research and development effort directed tothe definition and manufacture of carriers for functional substances fora wide variety of commercial applications. Ideal carrier compositionsare those that inherently exhibit high capacity for carrying/containmentof functional substances, those which work to enhance or prolong thefunctionality of the contained or carried substance, and those which canbe economically manufactured to meet the unique specifications requiredfor each targeted application. Such are the characteristics of thecomposition of the present invention.

In accordance with this invention, there is provided a versatile, lowcost porous composition having a high void volume and thus a highcapacity for containment for functional substances. The compositioncomprises free flowing aggregates of discrete particles, most preferablystarch granules, bound together with a binder at their points of contactin the aggregate. The use of starch granules as the particle componentof the present carrier composition provides surprisingly uniformspherical aggregates ranging in diameter from about 15 to about 150microns, depending on the size distribution of the component starchgranules. The surfaces of the aggregated particles cooperate to definean intraaggregate reticular volume for releasable containment of thefunctional substances. Advantageously the physical/chemicalcharacteristics of the composition can be readily adjusted to meet thefunctional requirement of each targeted application by selection of theparticle and binder components.

The particle aggregate compositions in accordance with this inventionare prepared by forming a suspension of the particulate component in asolution of a binder and spray-drying the resulting suspension utilizingart-recognized spray drying equipment/technology. The particulatecomponents can be pre-treated to promote their compatibility with thetargeted functional substance and to impart other properties such ashardness and solubility characteristics appropriate for the contemplatedcarrier application. Further, the binder, typically a polymeric materialexhibiting affinity for the particle component, can be selectedaccording to its chemical and physical characteristics to optimizefunctionality of the particulate aggregates as a carrier in a targetedapplication. Thus the binder component can be selected with view of itssolubility, its chemical reactivity, for example, its bioerodability orbiodegradability, as appropriate to optimize functionality of theparticle aggregates of the invention. Finally, the present aggregatecompositions can be coated to provide additional functionality.

Functional substances can be easily introduced into the reticular volumeof the present porous aggregates. The high intraaggregate reticularvolume and high internal surface area of the present aggregatecompositions allow high loading of functional substances. The loadedporous aggregate compositions of this invention are free flowing powderswhich facilitate handling and mixing of the functional substance inproduct formulations and further provides a matrix for sustained orprolonged release of the carried functional substance. Additionally, itis contemplated that the particle aggregates in accordance with thisinvention will exhibit functionality independent of their use as acarrier for functional substances. Thus, they may be used in preparedfoods that require minute gritty character, either in mouth feel or inappearance. The particle aggregate composition in accordance with thisinvention has utility in the areas of food/nutrition, the preparation oftopical creams and lotions, deodorant/antiperspirants, cosmetics,agricultural products, and products for human and veterinary medicine.The present compositions can be designed to enhance and prolong thefunctional characteristics of contained functional compositions.Alternatively, the present composition can function to protect thecontained functional substance from premature degradation. For example,orally administered pharmaceutical compositions can be formulatedwith/in the aggregate compositions, preferably the granular starch-basedcompositions of this invention, to provide an enteric formulation whichfunctions to protect the active substance from the acid/digestiveconditions of the stomach and thereafter release the active substance inthe small intestine.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, there is provided a porousaggregate of discrete particles. The particle components of the presentaggregates typically have an average particle size of about 1 to about100 microns in their largest dimension. The particles are bound togetherwith a binder, at least at their points of contact in the aggregate, sothat the surfaces of the aggregated particles cooperate to define anintraaggregate reticular volume into which one can introduce functionalsubstances. The contained substances are capable of being released fromthe aggregate over a period of time commensurate with the diffusion ofthe contained functional substance into the surrounding environment withor without the cooperation of disintegration of the aggregate due tosolubilization or bioerosion/biodegradation of the binder or particulatecomponents of the aggregate.

The present particle aggregate composition is prepared quite simply andeconomically by suspending the particles in a solution of a suitablebinder and thereafter spray-drying the particulate suspension usingart-recognized spray-drying methodology/equipment. Optionally theparticle aggregate can be further processed by applying a polymercoating to the surface of the particulate aggregate after it is formed,either before or after a functional substance is introduced into theintraaggregate reticular volume. The coating process can be carried outusing spray coater equipment such as that used in tablet manufacture orin art-recognized fluidized bed-type coating equipment.

The discrete particles utilized in preparing the present porousaggregates are preferably selected to have an average particle size ofabout 1 to about 100 microns, more preferably about 1 to about 75microns, in their largest dimension. Exemplary of such particulatematerial useful in accordance with this invention are starch granules,particulate cellulosic materials such as micronized wood pulp or gainhulls, and particulate polymer materials such as those sold in the artas flattening agents for coating compositions, for example, Pergopak®polymer particles sold by Martinswerk. Preferred particulate materialsfor use in accordance with this invention are granular starches,including native granular starches from various vegetable sources suchas corn, barley, rice and wheat, which are known for their somewhatlarger starch granules, and as well, from vegetable sources known toproduce small starch granules such as amaranth, quinoa, dasheen, cowcockle, pigweed and Chinese taro. Small starch granules such as thosefrom amaranth and the size classified small starch fraction of wheatstarch are particularly preferred for use in accordance with thisinvention.

Granular starches, presumably due to their inherent spherical orspheroidal structure, form substantially spherical granular starchaggregates when used as the particulate component of the aggregatecompositions of the present invention. Optionally the granular starchcomponent of the preferred embodiments of present composition cancomprise chemically modified granular starches including granularstarches that have been rendered microporous by being subjected topartial hydrolysis with acid or enzyme. Further, granular firmness andsurface characteristics can be advantageously adjusted by pretreatmentof the granules intended for use in the present particle aggregatecompositions. Thus, for example, a greater degree of structuralintegrity and firmness can be introduced by pre-treating granular starchwith an effective amount of a bifunctional starch-reactive chemicalcross-linking agent. Any of a wide variety of art-recognized starchcross-linking agents, including those recognized as food-acceptable bythe Food and Drug Administration, can be used. Suitable cross-linkingagents include phosphates such as sodium trimetaphosphate, dicarboxylicacid derivatives, particularly C₂ -C₆ dicarboxylic acids, includingmaleic and glutaric acid, phosphorous oxychloride, epichlorohydrin, andβ,β-dichlorodiethylether. Granular starches are rendered more resistantto mechanical damage, to swelling and to dissolution with increaseddegree of cross-linking.

Further the surface characteristics of the granular starches for use inpreparation of the particle aggregates of the present invention, andthus the surface characteristics and absorptive capacity of theresultant aggregates, can be affected by other surface modification ofthe granular starch component. Thus granular starches intended for usein accordance with the present invention can be pre-treated withsurface-modifying agents to enhance granule compatibility withfunctional substances targeted for use with the porous particleaggregate. If the substance to be introduced into the reticular volumeof the particle aggregate composition has a predominant lipid character,the starch granules can be treated to render their surfaces morelipophilic. Thus, the granules can be surface treated with solutions ofamphophilic polymers, or the surfaces of the granules can be chemicallyderivatized, for example, by reacting the granules with stearyl- oroctyl-succinic acid anhydride. The granule surfaces are thereby renderedmore lipophilic and more compatible with functional substances having apredominant lipid character. Surface characteristics of the granularstarch component of the present compositions can also be modified forenhanced lipophilicity by pretreatment with esterifying agents such aslong chain fatty acids or derivatives thereof, or by etherification withlong chain fatty halides. Treatment with acetic anhydride will alsoprovide some lipophilic character to the granules, but a higher level ofderivatization is required.

The porous particulate aggregates in accordance with this invention areprepared by spray-drying a slurry of particles in a solution of a bindercomponent. The chemical nature of the binder is not critical, except tothe extent the binder should exhibit some threshold affinity for thesurface of the discrete particles so that it can operate to bind theaggregated particulates together at least at their points of contactduring the spray-drying process. Inherently, too, the binder componentmust have some threshold solubility in the liquid used to suspend theparticle component prior to the spray drying operation. That liquid istypically water, however, other liquids such as C₁ -C₆ alcohols, ethersand ketones may also be employed where the targeted functionality of theporous particle aggregate requires use of a binder not having thethreshold solubility in water. Preferably, however, the porous particleaggregates in accordance with this invention are prepared byspray-drying aqueous suspensions of discrete particles, preferablystarch granules, suspended in an aqueous solution of a binder.

There exists a wide variety of suitable binders that can be used in theformation of the present particle aggregates. They are, most typically,polymer compositions exhibiting the requisite degree of solubility inthe liquid carrier for the particle suspension spray dried to form theporous aggregates. The polymer materials can be water soluble, waterinsoluble, biodegradable/bioerodable, not biodegradable, natural,synthetic, or semisynthetic--the binder to be selected for anyparticular application being dependent on the desired functionality,chemical/physical stability and release characteristics of the targetedaggregate in accordance with this invention. Preferred binders for usein accordance with this invention are biodegradable polymers such aspolysaccharides including gums such as guar and locust bean gums,pectins, agar, alginate, gelatin, dextrins, dextran and derivatizedstarches and cellulosic materials such as carboxymethyl cellulose,hydroxymethyl cellulose, hydroxypropyl cellulose, and the like,proteins, particularly proteins other than those endogenous to thestarch granules used to form the aggregates, and polyesters.Polysaccharides are preferred binders for use in accordance with thepresent invention. The binder components can also be non-biodegradable,synthetic or semi-synthetic polymers, such as polyvinyl alcohol,poly-N-vinyl-2-pyrrolidone, and polymers or co-polymers of acrylic ormethacrylic acid and amide derivatives thereof, includingpolyacrylamide.

In accordance with the method embodiment of the present invention,porous aggregates of discrete particles are prepared by spray-drying asuspension of such particles in a solution of a suitable binderutilizing conventional spray-drying equipment/conditions. One limitationon the spray-drying process is the stability of the particle component.Specifically, when spray-drying granular starch suspensions in aqueousbinder solutions, it is very much preferred that the temperature in thespray-drying operation is not so high as to effect gelatinization of thestarch granules.

The binder component is typically functional at very low levels, mostpreferably about 0.1 to about 2% by weight of the solution/suspension.The particulate component usually constitutes between about 2 and about20 weight percent of the suspension prepared for spray-drying inaccordance with this invention to produce the present porous particleaggregates.

The porous particle aggregate composition in accordance with thisinvention exhibits physical and chemical characteristics dependent onthe constituent discrete particles and the binder component. Theaggregates range in size from about 10 microns to about 250 microns,more typically between about 15 and about 150 microns, most typicallybetween about 10 and about 50 microns. The size and shape of the presentparticle aggregates depend significantly on the shape and particle sizedistribution of the component discrete particles. Thus when thepreferred particle component, starch granules, are utilized to form thepresent porous aggregates, the aggregates assume a remarkably uniformspherical shape with the individual granules being bound together withthe binder components at their points of contact. The surfaces of theaggregated starch granules cooperate to define an intraaggregatereticular volume for releasable containment of a functional substance.

The size, shape and particle size distribution of the porous particulateaggregates prepared in accordance with this invention are also dependenton the conditions selected for the spray-drying operation. Conventionalspray-drying parameters, however, used to form the preferredstarch-granule-based aggregates in accordance with this invention,produce surprisingly uniform porous, spherical aggregates.

The porous particle aggregate composition of the present invention isadvantageously utilized as a carrier for a wide range of functionalsubstances. The term "functional substances" as used herein to describethe present invention refers to any compound or composition whichinherently possesses biological or other functional activity and whichexhibits such activity to achieve some useful result when applied orused in a manner adapted to take advantage of such activity. Exemplaryof such substances which can be absorbed into the intraaggregatereticular volume of such aggregates in accordance with such inventionare salad oils, flavors, insect repellants, insecticides, herbicides,perfumes, moisturizers, soaps, antiperspirants, waxes, body creams andlotions, fertilizers, minerals, vitamins, bacteriostats, and therapeuticdrug substances. Such functional substances can be absorbed or otherwiseintroduced into the porous aggregates of the present invention either byspraying solutions of such substances onto the prepared aggregates,adding such substances to the particle slurries prior to the spraydrying process, or by adding the aggregates to solutions of saidsubstances wherein the solvent for such substances is selected so as notto prematurely dissolve or otherwise disrupt the aggregate bindercomponent. The product aggregates containing functional substanceswithin the reticular volume defined by the surfaces of the aggregatedparticles can be isolated in the process utilizing art-recognizedtechniques such as filtration, centrification, air classification anddrying. The degree of loading of functional substances into the porousaggregates can be controlled in part by adjusting the concentration ofthe functional substance in the solutions used to load the aggregatematrices. Higher concentrations of the loaded material can be achievedusing more concentrated solutions of the substances and by repeating theloading procedure. Preferably the substances are introduced into theporous particulate aggregates either as a component of the spray driedslurry or suspension used to form the aggregates, or as in solution inan inert, relatively low boiling solvent, which can be removed byevaporation following loading of the aggregate matrix. A hydrophobicliquid, such as a flavor oil, can be loaded into the aggregate by simplyallowing the oil to soak into the porous aggregate.

The release characteristics and other physical properties of theparticulate aggregates in accordance with this invention can be furthermodified by coating the aggregates following their preparation with asolution of the same or a different binder/polymer. The coatingoperation is preferably carried out after the loading of the aggregateswith the desired functional substance. The coating operation can beaccomplished simply by spraying the porous particulate aggregatecomposition with a dilute solution of a polymer which may be the same ordifferent from that used as the binder component of the aggregatecomposition. The coating operation can be conducted in spray coater typeequipment such as that used for conventional tablet coating operationsor in conventional fluidized bed-type coating equipment. The polymerutilized in the optional aggregate coating operation can be selected tooptimize the targeted functionality of the aggregate carriercomposition. Suitable polymers include any of those mentioned above asbinder components and as well other art-recognized coating compositionsutilized in conventional tablet coating applications.

The porous particle aggregates in accordance with this invention can beused, without added functional substances, as a bulking agent or toimpart other desirable organoleptic characteristics such as mouth feel,in various prepared foods. Preferably, however, the present compositionsare used as carrier and excipient for functional substances to enhanceor prolong substance functionality. Thus, for example, the presentcomposition can be utilized as a carrier for functional liquids,essentially converting them in form to free-flowing powders which can beused as a substitute for such functional substances in compositions topromote and/or prolong substance functionality. The carried or containedfunctional substances are released from the porous particulateaggregate, by simple diffusion, or upon mechanical compression or bychemical degradation or simple dissolution of the binder and/or particlecomponents. Thus, for example, it has been found that granular starchaggregates in accordance with this invention utilizing a guar gum orcarboxymethyl cellulose binder can be "loaded" with a flavor oil andused as a component of chewing gum to prolong flavor release.

The following Examples are presented to illustrate the present inventionand should not in any way be construed as a limitation thereof.

EXAMPLE 1

Amaranth starch granules are slurried in a solution of about 0.5 toabout 1% by weight of a commercial high viscosity guar gum to producestrong spherical aggregates. The spherical aggregates range from about10 to about 30 microns in diameter.

EXAMPLE 2

Granular amaranth starch was suspended in a 0.1% guar gum solution andspray dried to produce spherical aggregates having a size range of about10 to about 30 microns. The aggregates were dispersed in mint oil,centrifuged and thereafter washed with ethanol in a fritted glass funneland dried to provide a mint oil loaded aggregate composition containingabout 35% by weight mint oil. The mint oil loaded starch aggregatecomposition was then spray coated with a 0.5% solution of guar gum. Thecoated spheres were essentially without odor, but released mint oil andodor when they were rubbed on a glass plate with a metal spatula.

EXAMPLE 3

A slurry of rice starch in a 1% aqueous solution of locust bean gum isspray dried to produce porous spherical aggregates having a highintraaggregate reticular volume.

EXAMPLE 4

The small granular fraction of wheat starch obtained by sizeclassification of native wheat starch is slurried in an aqueous mediumcontaining medium viscosity methylhydroxypropyl cellulose and spraydried to produce substantially spherical aggregates of small wheatstarch granules. The aggregates are dispersed in mint oil, centrifugedand thereafter washed with ethanol in a fritted glass funnel and driedto provide a mint oil loaded aggregate composition containing about 48%by weight mint oil. The mint oil loaded starch aggregate composition wasthen spray coated with a 0.5% solution of gelatin. The coated sphereswere essentially without odor, but released mint oil and odor when theywere rubbed on a glass plate with a metal spatula.

EXAMPLE 5

A suspension of 10 grams of a micronized wood pulp having an averageparticle size of about 5 to about 15 microns in 150 ml of ethanolcontaining 1.5% by weight of poly-N-vinyl-pyrrolidone is spray-dried ina conventional spray dryer to produce porous aggregates. The aggregatecomposition is dispersed in an aqueous solution of a pesticide, filteredand dried to provide a pesticide loaded particle aggregate compositionin accordance with this invention.

EXAMPLE 6

Rice starch is slurried in 0.1% guar gum solution and spray dried toproduce a free-flowing powder comprising spherical aggregates about 30microns in diameter. The aggregates were non-hygroscopic and held theirspherical shape under normal processing. The disintegration of the ricestarch aggregates in water occurs over a period of time during which theintergranular binding gum is dissolved to allow disintegration of thespheres.

EXAMPLE 7

Commercial corn starch is dispersed into a 0.1% by weight solution ofcarboxymethyl cellulose and spray dried to yield aggregates of granularcorn starch in accordance with this invention.

EXAMPLE 8

Rice starch is suspended for 5 minutes in a 0.2% solution of 20 DE(dextrose equivalent) starch dextrin and spray-dried at 120° C., spraynozzle setting to produce well defined spherical aggregates of ricestarch.

EXAMPLE 9

Amaranth starch granules were suspended in a 0.1% sodium alginatesolution and spray dried to form an alginate-bonded granular aggregate.The product is sprayed or briefly washed with a 1% calcium chloridesolution to convert the sodium alginate binder to water insolublecalcium alginate. The resulting spheres exhibited enhanced stabilityunder aqueous conditions up to temperatures near the gelatinizationtemperature of the starch component. The spheres are spray coated with a1% sodium alginate solution and thereafter sprayed with calcium chloridesolution to increase physical stability and water resistance. Mint oilfilled/calcium alginate coated spheres are stabilized to oil leakagefrom the aggregate composition.

EXAMPLE 10

Aggregates of amaranth and wheat starch formed with either acarboxymethyl cellulose or locust bean gum binder are incorporated at 2%by weight into an ice cream composition prepared with but 50% of thenormal fat content, without compromise of taste and mouth feel.

EXAMPLE 11

The small granular fraction of wheat starch is slurried in an aqueoussolution of 0.1% gelatin and spray dried to produce porous,substantially spherical aggregates. The aggregate composition isslurried in an alcoholic solution of an orally effective antibioticfiltered and dried. The dried aggregates are coated in a fluidized bedcoating machine with a 0.5% solution of ethyl cellulose of the type usedfor tablet coating. The coated aggregates are filled into capsules fororal administration.

I claim:
 1. A method for preparing porous aggregates of discreteparticles having an average particle size of about 1 to about 100microns in their largest dimension, said aggregated particles boundtogether in said porous aggregate with a binder not endogenous to thediscrete particles at least at their points of contact in saidaggregate, the surfaces of said aggregated particles cooperating todefine an intraaggregate reticular volume adapted for releasablecontainment of functional substances, said method comprising the stepsof forming a suspension of said particles in a solution of the binderand spray drying said suspension.
 2. The method of claim 1 furthercomprising the step of applying a polymer coating to the surface of theporous aggregates.
 3. The method of claim 2 wherein the discreteparticles are starch granules.
 4. The method of claim 2 wherein thepolymer coating is selected from the group consisting of apolysaccharide, a polyvinyl alcohol, a poly-N-vinyl-2-pyrrolidone, apolyacrylamide, and an acrylic polymer or copolymer.
 5. The method ofclaim 1 wherein the binder is a water soluble polymer.
 6. The method ofclaim 1 further comprising the step of introducing a functionalsubstance into the reticulate volume of the porous aggregate.
 7. Themethod of claim 6 further comprising the step of applying a polymersolution to coat the surface of the porous aggregate.
 8. The method ofclaim 7 wherein the discrete particles are starch granules.
 9. Themethod of claim 6 wherein the discrete particles are starch granules.10. The method of claim 1 wherein the discrete particles are starchgranules.
 11. The method of claim 1 wherein the discrete particlescomprise micronized cellulosic pulp.
 12. The method of claim 1 whereinthe discrete particles comprise synthetic polymer particles.
 13. Themethod of claim 1 wherein the binder is a biodegradable polymer.
 14. Themethod of claim 13 wherein the binder is selected from the groupconsisting of polysaccharides, proteins, and polyesters.
 15. Acomposition comprising an aggregate of discrete particles, saidparticles having an average particle size of about 1 to about 100microns in their largest dimension, said particles bound together with abinder not endogenous to the particles at least at their points ofcontact in said aggregate, the surfaces of said aggregated particlescooperating to define an intraaggregate reticular volume adapted forreleasable containment of functional substances.
 16. The composition ofclaim 15 wherein the particles comprise micronized cellulosic pulp. 17.The composition of claim 15 wherein the particles comprise syntheticpolymer particles.
 18. The composition of claim 15 further comprising afunctional substance in the reticulate volume of the aggregate.
 19. Thecomposition of claim 18 further comprising a polymer coating applied tothe surface of the aggregate.
 20. The composition of claim 2 wherein thepolymer is selected from the group consisting of a polysaccharide, apolyvinyl alcohol, a poly-N-vinyl-2-pyrrolidone, a polyacrylamide, andan acrylic polymer or copolymer.
 21. The composition of claim 15 whereinthe binder is a biodegradable polymer.
 22. The composition of claim 21wherein the binder is selected from the group consisting ofpolysaccharides, proteins and polyesters.